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Assessment of stress dependent elastic and permeability anisotropy using small cubic shale samples
- Publisher: European Association of Geoscientists & Engineers
- Source: Conference Proceedings, Sixth EAGE Shale Workshop, Apr 2019, Volume 2019, p.1 - 5
Abstract
Assuming transverse anisotropy, elastic properties and anisotropy of rock formations at depth are usually determined in the laboratory using a cylindrical rock sample, preferably plugged orthogonal to the symmetry plane. Such anisotropy is usually caused by bedding or laminations when present. On the other hand, permeability anisotropy evaluation requires a pair of companion samples cored along and orthogonal to the bedding plane. Shaly cores recovered from the well often experience drying, desiccation and stress-relief fracturing when not properly preserved, which makes it difficult to sub-sample standard-size cylindrical plugs. Therefore, we developed a new laboratory approach to test relatively small cubic rock samples without modifying existing pressure vessel equipment designed for standard cylindrical samples. This approach is tested on the Opalinus Clay and validated by comparing the elastic anisotropy results, in term of Thomsen's parameters, obtained on a parent specimen and three cubes extracted from it. We demonstrate that elastic anisotropy parameters inferred from the cubes are accurate as long as a small, homogeneous and representative samples of the formation can be recovered. If this condition is fulfilled, permeability anisotropy can also be successfully determined from small cubic samples using a dedicated permeameter designed for tight and low permeability cylindrical rock samples.